Overvoltage arresters

ABSTRACT

In an overvoltage arrester with an enclosed spark gap assembly of electrode disks and intermediate spacers of annular shape, at least one electrode of a pair has an excentrically located projection directed towards the other electrode of the pair and thereby defines in the ring space of the annular spacer a spot of initial response. The annular spacer has a lateral opening at a peripheral locality substantially opposite the excentric projection. The opening extends from the inner periphery through the spacer to the outer periphery thereof and has a small width in the peripheral direction as compared with the total extent of the periphery.

United States Patent Jalszt et al.

[ 1 May 2, 1972 [54] OVERVOLTAGE ARRESTERS [72] Inventors: Werner Jaksz t; Gert Schiele, both of 21 App1.N0.: 69,625

Cunningham ..3 15/36 Sarbach ..315/36 Primary Examiner-James D. Trammell Attorney-Curt M. Avery, Arthur E. Wilfond, Herbert L. Lerner and Daniel J. Tick [57] ABSTRACT In an overvoltage arrester with an enclosed spark gap assembly of electrode disks and intennediate spacers of annular shape, at least one electrode of a pair has an excentrically I [30] reign Application Priority Data located projection directed towards the other electrode of the Sept. 9, 1969 Germany ..P 19 46 4132 P and thereby defines in the s Space of the annular spacer a spot of initial response. Theannular spacer has a 52 us. Cl ..317/68 315/36 317/70 lateral Owning P P locality Substantially Opposite [51] lm. c1. ..H(12h 9/06 the Centric Projectim The P 6mm from the inner [58] Field or Search ..3l7/61 68 70 315/36- Periphery thmugh the spacer the Periphery hereof 3 l g/DIG and has a small width in the peripheral direction as compared with the total extent of the periphery. [56] References Cited 5 Claims, 3 Drawing Figures 7 UNITED STATES PATENTS 3,152,279 10/1964 Misare ..315/36 X I I5 3 .Z4

z 9 2 i L I -1 Patented May 2,1972 3,660,725

2 Sheets-Sheet l Plhntod May 2; 1972 2 Shuts-Shut 2 OVERVOLTAGE ARRESTERS Our invention relates to overvoltage arresters in which the housing of the arrester encloses a spark gap assembly composed of a stack of disk-shaped metallic electrodes and annular spacer members which separate each electrode from the one next adjacent in the stack, thereby formed a spark gap, the entire stack comprising a number of such gaps in series.

It is known to provide in such spark gap assemblies one or both electrodes of each pair with an excentric projection directed towardsthe other electrode of the pair so as to define the locality at which an arc will be initiated when the voltage impressed across the electrodes of the pair exceeds the rated minimum (threshold) value. In such assemblies the arc is supposedto wander on the electrode disks in order to avoid local over-heating as may cause the formation of fusion beads which are apt to change the critical threshold voltage of the arrester (German Pat. No. 933,823). On the other hand, the arc must not travel beyond the edge of the electrode disks because this may cause uncontrollable short circuiting of the spark gap. For that reason, the annular spacer member which, together with the electrode disks, encloses the arcing space, constitutes a desired limit for the migration of the are.

It is an object of our invention to increase the current switching ability of overvoltage arresters and spark gaps of the above-mentioned type.

Another object of the invention, akin to the one just mentioned, is to afford reducing the resistance of the usually voltage-dependent resistors connected in series with the spark gap assembly, such resistance reduction being made possible by the ability of the arrester to interrupt higher current intensities at a given voltage.

To achieve these objects and in accordance with our invention we provide in an overvoltage arrester of the abovedescribed type the annular spacer members in the spark gap assembly with a lateral opening which at a peripheral locality opposite the excentric electrode projection and hence opposite the locality of the initial breakthrough arcing, the lateral opening having a small size in comparison with the total periphery of the annular spacer member.

In contrast to other known spark-gap assemblies (US. Pat. No. 2,807,751), the arc chamber in assemblies according to the invention is virtually fully enclosed so that external discharges are avoided. At the same time, the lateral opening in an assembly according to the invention causes the occurrence of gas pressure oscillations excited by the arc discharge within the annular space of the spacer member. The oscillating gas volume drives the arc so intensively that a considerable increase in switching ability and hence an interruption of higher current intensity is attained.

According to a preferred embodiment of the invention, the spacer members in the voltage arrester are given an approximately Z-shaped cross-section. That is, the cross-section forms a step configuration so that an outer ring portion and an inner ring portion are formed, both merging with each other. The electrode disks are placed upon the axial faces of the inner ring portion; and the lateral opening is preferably designed as a recess which extends through the entire crosssection of the ring inner portion. With the spacer members stacked into each other, each inner portion is surrounded by the outer portion of the next adjacent spacer member. The an- I nular outer portion thus forms a mechanical reinforcement for the inner portion in the vicinity of the lateral opening. In many cases this pennits the application of higher stresses than with a ring member which is completely interrupted at one locality of the periphery. Furthen'nore the Z-shaped cross-section of the spacer permits an improved voltage distribution at the spark gap assembly as will be explained with reference to the embodiment described hereinafter.

Voltage arresters for high voltages, for example 20 kv., require stack assemblies of spark gaps with more than two electrodes. In such stack assemblies according to the invention, the lateral openings are preferably located at respectively different points along the stack periphery. It is of advantage to employ symmetrical arrangements in order to reliable secure maximal break-through voltages. For'example, each opening may be angularly displaced C. relative to the nextfollowing opening so that each two openings on the generatrix line of the cylindrical electrode stack are spaced from each other by the height of 2 individual spark gaps.

The invention will be further described with reference to an embodiment of an overvoltage arrester according. to theinvention illustrated by way of example on the accompanying drawings in which FIG. 1 is a longitudinal section of a complete overvoltage arrester,

FIG. 2 is an axial section through part of the same arrester; and

FIG. 3 is a cross-section taken along the line of Ill-Ill in FIG. 2.

As illustrated in FIG. 1, the overvoltage arrester l'comprises a cylindrical tubular housing 2 with skirts 3. The housing consists of porcelain or the like ceramic material and is covered on both ends by metallic armatures 4 and 5, each shaped as a cap and provided with a central electric terminal bolt. The tubular housing 1 encloses a stack of resistors 7 and spark gap members 8 which are all electrically connected in series. Attached to the lower end of the arrester is a fastening clamp 10 which is to be grounded, the terminal bolt of armature 5 being likewise connected to ground potential. The high voltage to be limited by the arrester is connected to the terminal'bolt of the top cap 4. The resistor disks 7 consist essentially of silicon carbide (SiC The individual spark gap members 8 conjointly form a cylindrical stack (FIG. 2). Each two electrode disks l4 and 15, both consisting of the same material such as brass and both having the same planar shape and size, are separated from each other by a weakly conductive spacer member 13. This member has a generally Z-shaped cross-section so that the member, seen in cross-section, forms a step configuration with a ring-shaped inner portion 16 merging with a ring-shaped outer portion 17. The outer portion extends about the inner portion of the next adjacent spacer member. The spacer members 13 are preferably. made of ceramic material or of synthetic plastic, for example in epoxy resin. These insulating materials are made weakly conductive by an addition of about 20 percent by weight of silicon carbide, the resistivity of the spacer members 13 being preferably about 10 Ohm.cm.

Each electrode disk 14 and 15 is provided with an excentrically located semisspherical bulge 20. This bulge is made by pressing it out of the otherwise planar electrode disks or foils. Each bulge 20 constitutes a projection directed toward the adjacent electrode and located in the ring space of the spacer member. The projection thus defines a locality 21 where the first breakthrough arc will occur when the arrester responds to excessive voltage.

A small opening 23 is provided in the peripheral wall of the spacer member 13 at a locality situated opposite the excentric projection 20 and hence opposite the locality 21 of initial response, the opening 23 occupying substantially the same diametrical plane or range as the initial arcing locality 21. As apparent from FIGS. 2 and 3 the opening 23 is formed by a recess in the spacer member 13 extending over the entire cross-section of the ring inner portion 16. The width of the opening 23 in the peripheral direction of the spacer member is approximately as large as the diameter of the projection 20. Consequently, the opening 23 is approximately 2 to 5 times as large as the length of the breakthrough gap at the initial arcing locality 21, the drawing, for lucidity, being not true in this respect to actual measure.

The openings 23 of the stack 12 are uniformly spaced angularly by 180 along the peripheral surface of the stack. This symmetrical arrangement of the openings provides for the largest possible distance between each two sequential openings. Such an arrangement can be fixed, conjointly with providing for correct positioning of each projection 20 and appertaining opening 23 on the same diameter, by providing the stack assembly with mutually engageable tongue-and-groove means or the like. Such means facilitate assembling the arrester and permit securing the correct position between the initial arcing locality 21 and the gas blowout opening 23 in a nearly automatic manner. For example, a projection of the electrode, similar to those described above may engage the opening 23 which in this case perfonns the double function of also providing for relative positioning of the components. For example, in FIG. 2 the electrode is shown to have two bulges 24 which enter into the adjacent openings 23 for securing the proper position of the projections relative to the peripheral locality of the openings 23. To facilitate obtaining and securing the proper relative positioning, several bulges, bosses or projections similar to those shown in FIG. 2 may be distributed over the periphery.

Tests have shown that spark gap assemblies according to the invention result in a considerably improved are quenching action in overvoltage arresters of the enclosed type described above. This is due to the fact that the arc initiated in the gap 21 is not only driven off the projection 20 by a pressure reflection caused by the spacer wall area adjacent to the projection 20, but is also subjected to subsequently occurring pressure oscillations which apply motive forces to the arc, these oscillations of the gas column being excited by the arc discharge on account of the opening 23.

In overvoltage arresters according to the invention, the spacer members 13 have high mechanical strength since the opening 23, although extending over the area of the ring inner portion 16, is bridged by the ring outer portion 17 so that the spacer member 13 as a whole has a closed annular configuration. The spacer member therefore reliably surrounds and encloses the are even in the event of considerable pressures. In the event of rapidly successive unipolar voltage surges in the inner space of ring portion 16, the weakly conducting material of the spacer member 13 has the efiect of equalizing any build-up of electric chargers between adjacent electrodes so that stray effects of the overvoltage responded to, are considerably reduced.

Upon a study of this disclosure, it will be obvious to those skilled in the art that with respect to design, configuration and other details our invention permits various modifications and may be given embodiments other than illustrated and described herein.

We claim:

1. In an overvoltage arrester comprising an enclosed spark gap assembly having disc-shaped metallic electrodes and an annular spacer member between each two adjacent ones of said electrodes, said electrodes and said spacer member having a common central longitudinal axis, at least one of said electrodes having inside the ring space of said annular spacer member a projection located eccentric of said axis near the wall of said spacer member, said projection being directed toward the other electrode and defining therewith a spot of initial response of the spark gap, and said annular spacer member having a lateral opening, said excentric projection and said opening being at mutually opposite sides of said axis, said opening extending from the inner to the outer periphery of said spacer member and having a small width relative to the total size of the periphery.

2. In an overvoltage arrester according to claim 1, said annular spacer member having an approximately Z'shaped cross-section forming an outer ring portion and an inner ring portion of which the inner one has a smaller diameter than the outer portion and which are integral with each other, said electrodes being in contact with the annular front faces of said inner ring portions, and said lateral opening extending through the cross-section of said inner ring portion.

3. In an overvoltage arrester according to claim 1, said spark gap assembly comprising a multiplicity of said electrodes and spacer members stacked alternately upon each other, said respective lateral openings of said spacer members in said stack being angularly spaced from each other relative to the periphery of said stack.

4 In an overvoltage arrester according to claim I, said spark gap assembly comprising mutually engageable tongue-andrecess means which fix the position of said spacer members relative to the adjacent one of said projection-carrying electrodes respectively.

5. In an overvoltage arrester according to claim 4, said tongue-and-recess means comprising a projection on an electrode and one of said lateral openings with which said latter projection is engageable. 

1. In an overvoltage arrester comprising an enclosed spark gap assembly having disc-shaped metallic electrodes and an annular spacer member between each two adjacent ones of said electrodes, said electrodes and said spacer member having a common central longitudinal axis, at least one of said electrodes having inside the ring space of said annular spacer member a projection located eccentric of said axis near the wall of said spacer member, said projection being directed toward the other electrode and defining therewith a spot of initial response of the spark gap, and said annular spacer member having a lateral opening, said excentric projection and said opening being at mutually opposite sides of said axis, said opening extending from the inner to the outer periphery of said spacer member and having a small width relative to the total size of the periphery.
 2. In an overvoltage arrester according to claim 1, said annular spacer member having an approximately Z-shaped cross-section forming an outer ring portion and an inner ring portion of which the inner one has a smaller diameter than the outer portion and which are integral with each other, said electrodes being in contact with the annular front faces of said inner ring portions, and said lateral opening extending through the cross-section of said inner ring portion.
 3. In an overvoltage arrester according to claim 1, said spark gap assembly comprising a multiplicity of said electrodes and spacer members stacked alternately upon each other, said respective lateral openings of said spacer members in said stack being angularly spaced from each other relative to the periphery of said stack. 4 In an overvoltage arrester according to claim 1, said spark gap assembly comprising mutually engageable tongue-and-recess means which fix the position of said spacer members relative to the adjacent one of said projection-carrying electrodes respectively.
 5. In an overvoltage arrester according to claim 4, said tongue-and-recess means comprising a projection on an electrode and one of said lateral openings with which said latter projection is engageable. 